Superconductivity is a phenomenon occurring at very low temperatures in many electrical conductors, in which the electrons responsible for conduction undergo a collective transition to an ordered state, of which superconductivity is a characteristic. This ordered state exhibits several unique and remarkable properties: disappearance of resistance to the flow of electric current, appearance of a large diamagnetism and other unusual magnetic effects, substantial alteration of many thermal properties, and the occurrence of quantum effects otherwise observable only at the atomic and subatomic level. The temperature below which a conductor begins to exhibit superconductivity is called the transition temperature or "critical temperature," usually designated T.sub.c. Below the critical temperature, electrical resistance of low-temperature superconductors drops sharply to levels at least 10.sup.12 times less than at normal temperatures. In high-temperature superconductors in the microwave and millimeter wave regions, the resistance drops sharply to levels on the order of 10.sup.3 to 10.sup.4 times less than at normal temperatures.
Other phenomena beside the disappearance of electrical resistance are displayed by superconductors. One of these is the Meissner-Ochsenfeld effect, in which an applied magnetic field is excluded from the interior of the superconductor. As long as the magnetic flux in a superconductor is low, the superconductor will remain completely superconducting in an applied magnetic field. If the magnetic field becomes too large, however, the superconductor will become partially or totally normal. That is, when the magnetic field exceeds a "critical field," designated H.sub.cl, the superconductor gradually reverts to the normal state and its resistance to electric current rises sharply.
Related to the Meissner-Ochsenfeld effect is the phenomenon of effective penetration depth. The way in which a superconductor excludes from its interior an applied magnetic field smaller than the critical field H.sub.cl is by establishing a persistent supercurrent on its surface and inside the material to the effective penetration depth which exactly cancels the applied field inside the superconductor. This current effectively flows in a very thin layer of thickness .lambda., which is called the effective penetration depth. The external magnetic field also penetrates the superconductor within the penetration depth. Lambda depends on the material and on the temperature, and is typically very small, on the order of 2000 to 5000 Angstroms.
The existence of the critical field leads to another property of superconductors which is of importance. A supercurrent flowing in a superconductor will itself create a magnetic field, and this field will drive the superconductor normal at some critical value of the current, called the critical current density, designated J.sub.c. When the current in the superconductor exceeds the critical current density, the superconductor becomes normal and its resistance increases sharply.
These phenomena of superconductors can be put to practical applications. For example, a superconductor can be used as a switching device if it can be driven from the normal to the superconducting state and back again as desired. One way to change the state of a superconductor from superconducting to normal is to change the critical field. This approach is disclosed in U.S. Pat. No. 3,327,273, which discloses a gate element composed of a thin-film superconductor whose resistance is controlled by the application of an external magnetic field. By controlling the external magnetic field, the gate element can be driven from the superconducting to the normal state, and vice-versa.
These phenomena have also been exploited to create a variable resistance superconducting device, as shown in U.S. Pat. No. 2,978,664. This patent shows a tapered conductor of superconducting material which operates partially in the superconducting state and partially in the normal state. By tapering the conductor, there will eventually be a point at which the current density through it exceeds the critical current density, at which point the conductor becomes normal. By locating tap points along the tapered length of the conductor, different resistance behaviors can be obtained.
The present invention differs from the approaches shown in these patents in that the invention does not require any complex geometries or associated field generating apparatus such as coils and windings. The present invention provides a simple superconducting non-linear device that can be used for switching, limiting, mixing, detecting and other applications.